首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   108585篇
  免费   2097篇
  国内免费   908篇
测绘学   2787篇
大气科学   7824篇
地球物理   22318篇
地质学   37956篇
海洋学   9181篇
天文学   24490篇
综合类   303篇
自然地理   6731篇
  2021年   961篇
  2020年   1078篇
  2019年   1103篇
  2018年   2558篇
  2017年   2414篇
  2016年   3092篇
  2015年   1892篇
  2014年   3087篇
  2013年   5710篇
  2012年   3129篇
  2011年   4243篇
  2010年   3669篇
  2009年   4819篇
  2008年   4450篇
  2007年   4267篇
  2006年   4024篇
  2005年   3361篇
  2004年   3290篇
  2003年   3052篇
  2002年   2854篇
  2001年   2660篇
  2000年   2482篇
  1999年   2171篇
  1998年   2216篇
  1997年   2134篇
  1996年   1732篇
  1995年   1743篇
  1994年   1554篇
  1993年   1431篇
  1992年   1389篇
  1991年   1316篇
  1990年   1467篇
  1989年   1291篇
  1988年   1191篇
  1987年   1390篇
  1986年   1187篇
  1985年   1507篇
  1984年   1688篇
  1983年   1605篇
  1982年   1544篇
  1981年   1398篇
  1980年   1265篇
  1979年   1193篇
  1978年   1200篇
  1977年   1085篇
  1976年   1066篇
  1975年   962篇
  1974年   1026篇
  1973年   991篇
  1972年   631篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
51.
52.
53.
Long-term changes in growth of Norwegian spring-spawning herring   总被引:3,自引:1,他引:2  
  相似文献   
54.
Recent findings by the MER rover opportunity confirming the presence of iron minerals that can only be formed in the presence of water emphasize the study of analogous environments to Mars on Earth. The study of chemolithoautotrophic communities living in acidic iron-rich habitats is highly relevant in order to identify Mars analog environment-specific biomarkers. Iron oxidizing bacteria like Leptospirillum ferrooxidans and Acidithiobacillus ferrooxidans have ways of life for which it is feasible to identify a past or present hypothetical niche on Mars. We have developed a strategy for biomarker identification based on: (i) search for biosignatures on acid and metal-rich environments; (ii) development of an immunosensor microarray; and (iii) integration into an instrument for autonomous and remote operation. The instrument that we have built, called Signs Of LIfe Detector (SOLID), is capable of processing a variety of samples for the detection of specific biomarkers. Antibodies against several bacterial strains have been developed and tested in a microarray biosensor on SOLID. Tests with field samples have been successfully performed, allowing the detection of L. ferrooxidans, A. ferrooxidans present in sediment samples.  相似文献   
55.
We consider the particular solutions of the evolutionary system of equations in elements that correspond to planar and spatial circular orbits of the singly averaged Hill problem. We analyze the stability of planar and spatial circular orbits to inclination and eccentricity, respectively. We construct the instability regions of both particular solutions in the plane of parameters of the problem.  相似文献   
56.
The Hubble Space Telescope (HST), a large optical telescope having an aperture of 2.4 meters and a length of 8.8 meters, is being developed by the National Aeronautics and Space Administration. This telescope will be placed into earth orbit by the space shuttle. Astrometric observations with the HST are made using a Fine Guidance Sensor which is capable of measuring the position of one object relative to another with an accuracy of ±0.002 arcseconds. The astrometric user of HST will be provided with an Astrometric Data Reduction Software package (ADRS). The variety of astrometric problems to be investigated with HST is discussed.  相似文献   
57.
The chemical composition of several thermal springs around Changbaishan area has been investigated. Cenozoic basaltic rocks are widely distributed in Northeast China and geothermal characteristics have been described. About one hundred hot springs exist around Changhaishan Volcano at the border between China and D.P.R.Korea with high temperature about 82~C. The pH values of the spring water range from 6.9 to 7.1 and the total flow rate is about 4.8 L/sec. The chemical composition of the thermal springs is sodium carbonate; the high-mineral contents of thermal water are believed to have medicinal properties. Bathhouses are already built along the hot springs to take the advantage of the supposed healing properties. The high quality of those hot springs is believed to be utilitized for mineral water. The chemical equilibrium temperatures were estimated at about 160℃ based on the Na-K-Ca geochemical thermometer.  相似文献   
58.
59.
60.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号